In recent years, vessels have been required to improve the efficiency of a propulsive apparatus configured to generate a propulsive force, in view of shortage of energy resources or the like. Under the present circumstances in which shipboard devices have been changed from hydraulically-operated devices to motor-operated devices, a propulsive apparatus that generates the propulsive force with a motor has been increasingly employed rather than a conventional propulsive apparatus directly coupled to a main engine. For example, U.S. Pat. No. 6,692,319 discloses a propulsive apparatus for submarines/submersible vessels that includes a ring-shaped motor rotor provided with propeller vanes protruding radially inward thereof. According to this propulsive apparatus, water is ejected by the rotation of the propeller vanes through a space defined by the ring-shaped motor to generate a propulsive force.